Here is a great explanation on how to undercoat and prime for painting your car that I read and thought was a really good write up and worth sharing if you are planning this type of work on your car.
Applying Undercoat Paint to Motor Cars.
Undercoat spraycans are useless except for light touches when sanding primer later where you touch bare metal.
They are mainly thinners and have SFA coverage.
Acrylic primer wether aerosol can or spraygun is not completely waterproof and panel can rust within a few weeks. or even tho unseen , rust under the primer and you still paint over it.
The etch primer will help but if a single pac like wattly super etch, needs a couple of good coats to give some protection.
You need aprox 3 coats of acrylic primer at 40+ microns [full wet coat with between 100 to 150 %thinners] to give a reasonable protection in situation that the panel wont be wetted too much or prolonged [weeks-months] high humidity.
I know some will say they last ok longer than this but no point risking having to strip panels to bare metal again.
You will get some ghosting as the bog is more absorbent or the metal may have been cold.
As long as you have about 60 microns dry film build it shouldnt matter. As a test. dry sand a little with 180 when paints dry and apply another couple coats with spraygun. This coat should come out fairly even as the surface is sealed.
Go a good 3 coats of primer [acrylic. ] fully dry for a day at least to allow for shrinkge. 2 wet coats with spray putty. Light coat of primer at any stage to act as a guide coat. Allow to shrink at least a couple of days at current temps.
1/2 sheet block sander -block panels with 180 dry.
Repeat till dead straight. Final sand w 240 dry
MUST have a couple of good coats of primer over this guide coat. Block again with colour sanding block [flexible rubber about 1cm thick] 400 grit dry then 600 final coat of primer applied wet as if 1st colour coat. then a denib w 600g colour
I dont like one-shot primer. Better results with primer then spray putty.
You dont want to be leaving a final build over 1mm of putty after sanding
I use a fine filler such as dolphin glaze [softest one] as a final bog. Easier to sand and feather out. Need to work within 1/2 mm or less before priming and spray putty. Otherwise you will risk shrinkage problems.
If you see swelling in original paint or paint edges at any stage, these MUST shrink back completely before proceeding to next stage. In cold weather this may be a week or two sometimes. If edges 'fry’, allow to dry. sand and reseal area with multiple light applications .
There are many other tricks /ways to deal with it, but this should get you started.
